1. Field of the Invention
The present invention relates to a pigment dispersion, a method for producing the same, and a pigment dispersion producing apparatus.
2. Related Background Art
An inkjet recording method has major features such as compactness and low power consumption as a direct recording method. The image quality and the printing speed have been rapidly improved owing to a reduction in nozzle size and the like.
The inkjet recording method is a recording method in which an ink fed from an ink tank is discharged from a nozzle to form an image on a recording medium. Such methods include a method in which an ink is heated by a. heater in a nozzle and thereby vaporized and foamed to discharge the ink from the nozzle, and a method in which a piezo element is vibrated to discharge an ink from a nozzle.
A dye aqueous solution is normally used for an ink for use in the inkjet recording method, and therefore there are cases where bleeding occurs when colors are superposed, or a phenomenon called feathering emerges in the paper fiber direction at a position of recording on the recording medium.
For the purpose of improving the above situations, use of a pigment dispersion ink is under consideration. For example, an ink having a pigment stably dispersed using a polymer dispersant is disclosed in U.S. Pat. No. 5,085,698. The pigment dispersion ink has been found to have considerably improved light-resistance, water-resistance and fixation characteristics compared with the case where a dye is used. However, a pigment that is used in the ink is generally obtained through a step of mechanically pulverizing an agglomerate of a commercially available pigment using a dispersing apparatus such as a ball mill. Therefore, typically, the average particle diameter of the pigment after the pulverizing step is greater than about 100 nm (0.1 μm), and further the particle size distribution of even a pigment having a relatively large particle size distribution is in the range of 80 to 180 nm.
In U.S. Application Publication No. 2002-040662 is disclosed a method in which fluids discharged from nozzles are collided in a reactor chamber to produce a pigment dispersion. The publication discloses a method in which suspensions including a crude pigment, an agglomeration stabilizing liquid and a liquid medium are discharged from nozzles situated oppositely in the reactor chamber and mutually collided to pulverize and disperse the crude pigment at the same time.
However, even in this method, further improvements for stably obtaining a pigment dispersion having a very small particle diameter are desired.
Aside from this, there is a method disclosed in Japanese Patent Application Laid-Open No. 2004-33901 as a method for efficiently producing a metallic colloid solution having a small particle diameter. The publication discloses a method for producing a metallic colloid solution by reducing a metal compound under presence of a polymer pigment dispersant, wherein the reduction is carried out in a microreactor. According to the method disclosed in the publication, a metallic colloid having a small particle diameter can be produced efficiently, but production of a pigment dispersion having a very small particle diameter is not disclosed in the publication. Furthermore, particular control of a state of a fluid introduced into the microreactor is not disclosed.
Aside from this, there is a method disclosed in Japanese Patent Application Laid-Open No. 2001-172532 as a method for producing an aqueous ink. The publication discloses that a hydrophobic dye is dissolved in a supercritical fluid, and the resultant solution is atomized and mixed with water to obtain a dispersion. According to the publication, an ink having a dye uniformly dispersed can be obtained. However, the publication discloses neither production of a pigment dispersion having dispersed a pigment having a very small particle diameter nor use of a microreactor.